The present invention relates to a metal halide lamp apparatus, and particularly to a metal halide lamp apparatus which comprises a metal halide lamp and a reflecting mirror, which produces a luminous intensity distribution within an optically limited region and which is used as a relatively small image light source.
Conventional small metal halide lamp apparatuses each comprising a reflecting mirror and a metal halide lamp having a luminous tube absent an outer bulb, both of which are integrally or detachably combined, are characterized by good color rendering and high luminous efficiency. Thus, such lamp apparatuses are used as light sources for overhead projectors, overhead-type liquid crystal projectors, liquid crystal projection televisions, moving picture projectors and so on, and the apparatuses are increasingly popularized.
Halogen lamps are generally used as light sources for the above projectors because it is desired to use light sources having good color characteristics for the above projectors. However, although halogen lamps have good color characteristics, the luminous efficiency thereof is as low as about 30 lm/W. A high-watt lamp must be thus used for obtaining a large illuminance on a screen. However, the use of a high-watt lamp has the problems that the size of an apparatus is increased due to the treatment of the heat generated from the light source and that a large quantity of heat is generated and cannot be easily treated.
On the other hand, metal halide lamps have luminous efficiency higher than that of the halogen lamps. If at least dysprosium halide is enclosed in a luminous tube, and if a metal halide lamp is operated at an increased vapor pressure of the halide and a decreased arc temperature, the emission of light within the red region is increased, and a spectral distribution having good color characteristics is obtained.
In conventional metal halide lamps used in the above projectors, for example, with rated lamp power of 150 W, a substantially spherical luminous tube 101 having an electrode spacing of 5 mm, the maximum outer diameter .phi.11 mm and the maximum inner diameter of .phi.8.8 mm is used, as shown in FIG. 1. Mercury and 150 torr of argon serving as auxiliary starting gas are enclosed in the luminous tube 101 so that a predetermined lamp voltage is obtained, and dysprosium iodide, neodymium iodide and cesium iodide are enclosed in an amount of 0.5 mg relative to the inner volume of the luminous tube of 0.4 cc at a ratio by weight of 4:2:3 to form a metal halide lamp.
The luminous tube 101 configured as described above has both end wires which are respectively connected to a nickel lead wire 102 and a base 103. A reflecting mirror 104 made of hard glass and having a cold mirror film provided on the surface thereof is provided so as to surround the luminous tube 101 coaxially therewith. One end of the lead wire 102 is led to the outside of the reflecting mirror 104 and connected to a terminal 105 to form a metal halide lamp apparatus 111.
When the metal halide lamp apparatus 111 configured as described above is used as a light source for a liquid crystal projector, for example, the metal halide lamp apparatus 111 is vertically placed with the bottom up for projecting an image on a screen 115 through a total reflection mirror 112, a liquid crystal panel 113 and a projection lens 114, as shown in FIG. 2.
In conventional metal halide lamps used in the above-described optical devices, as shown in FIG. 1, a reflecting-heat insulating film 106 is formed in a portion of the outer surface of a lamp in the vicinity of the electrode placed on the open side of the reflecting mirror 104, the other portion of the lamp outer surface having a clear surface. When such a metal halide lamp is disposed in the reflecting mirror 104 so that the clear surface faces the bottom of the reflecting mirror 104, as shown in the drawing, and when the lamp is switched on, assuming that the luminous portion of the arc of the luminous tube 101 is placed at the center of the luminous tube 101, the light emitted from the luminous portion to the clear surface passes substantially straight therethrough, without being scattered by the clear surface.
When the lamp is projected on a screen having an aspect ratio of 3:4, as shown in FIG. 3, through a lens system, the straight light emitted from the metal halide lamp through the clear surface is reflected from a portion of the reflecting mirror near the lamp and reaches the screen through the lens system. A large quantity of straight light passes through the clear surface, substantially without being scattered, and a large quantity of light emitted to a portion of the reflecting mirror 104 near the luminous tube 101 is reflected from the portion because of the low eccentricity of the portion of the reflecting mirror. When an illuminance distribution along the line A--A' on the screen was measured, the illuminance in a portion near the center is extremely high, while the illuminance in the peripheral portion is extremely low, as shown by a curve in FIG. 4. If an illuminance ratio in respect to the illuminance measured at each of measurement points 1 to 9 at the centers of the respective regions which are obtained by dividing the screen into 9 equal parts is expressed by the following equation: ##STR1## when a metal halide lamp comprising a luminous tube most of which has a clear surface is used, the illuminance ratio is as low as about 10%.
It is considered on the basis of experience that the illuminance ratio is preferably 30% or more from the visual viewpoint. The use of a metal halide lamp most of which has a clear surface, as described above, has the problem that irregularity is produced in the illuminance on the screen due to a low illuminance ratio, resulting in an unpleasant feeling.
On the other hand, it is thought that a frost portion is formed over the whole surface of the luminous tube in order to remove the unpleasant feeling caused by the irregularity in illuminance. The frost portion is formed on the outer surface of the luminous tube by satin treatment, i.e., sand blast processing, in which glass breads are sprayed on the clear outer surface of the tube having the thus-obtained frost portion, as shown in FIG. 5, about half of the light 122 emitted from the luminous portion at the center of the luminous tube travels as straight light 123 from the frost portion 121, the remainder light traveling from the frost portion 121 as light 124 scattered in the vicinity of the straight light 123.
When the luminous tube 101 in which the frost portion 121 is formed in the whole surface thereof except the thermal insulating film 106 is disposed in the reflecting mirror for projecting light on a screen, the straight light 123 in an amount of about half of the emitted light is applied to a portion of the reflecting mirror, which has relatively large eccentricity, reflected therefrom and reaches the vicinity of the center of the screen. Only a small quantity of scattered light 124 reaches the vicinity of the center and the peripheral portion of the screen. When the illuminance distribution along the line A--A' on the screen is measured, a curve b shown by a dotted line in FIG. 4 is obtained. As seen from the measurement of illuminance, although the provision of the frost portion permits an increase in the illuminance ratio, there is the problem that a desired illuminance cannot be obtained at the center of the screen because of a decrease in the overall illuminance, and that a large quantity of light is uselessly scattered.
Conventional metal halide lamp apparatuses also have the following problems: When the vapor pressure of the enclosed metal halide is increased by increasing the temperature of the luminous tube wall in a metal halide lamp, good color characteristics are obtained. However, when the metal halide lamp is vertically disposed and used, if a bare luminous tube absent an outer bulb is used, the temperature difference between the upper and lower portions of the inner surface of the quartz container which forms the luminous tube is increased, as compared with the case of a luminous tube with an outer bulb. If a desired vapor pressure is obtained by increasing the temperature of the lower portion (coolest portion) of the luminous tube containing a melted enclosed filling to substantially the same temperature as that of a lamp with an outer bulb, therefore, the temperature of the upper portion of the inner surface of the quartz container is excessively increased. Particularly, in the case of a lamp in which dysprosium is enclosed for improving the color characteristics, devitrification occurs in the upper portion of the luminous tube in an early stage.
In this case, although the overall luminous flux is only slightly changed, the color temperature of the color characteristics is decreased due to the heat insulating effect caused by the devitrification, and the illuminance on the screen is further decreased due to an increase in scattering caused by the devitrification. There are thus the problems that the screen is darkened or discolored, and that the lamp voltage is further increased.